Uranium is a naturally occurring radioactive element widely distributed among igneous rocks and oxide minerals. There are three naturally occurring uranium isotopes, 238U, 235U, and 234U. All three natural isotopes undergo radioactive decay by alpha emission accompanied by weak gamma radiation. Although all three uranium isotopes are present in groundwater, 238U and 234U predominate on an activity basis. Uranium ore, composed of uranium-containing minerals such as uraninite, O3O8, and carnotite, K2(UO2)2(VO4)2.3H2O, are a mixture of 238U, 235U, 234U, and decay progeny, and are chemically stable under reducing conditions. However, if oxidants are introduced to the surface of these minerals, oxidative dissolution occurs. Upon dissolution, uranium can be found in groundwater at elevated concentrations and can find its way into water supplies.
It has been estimated that 0.3-6% of all ingested uranium is absorbed and deposited in the bones, kidneys, liver, and other soft tissues (Taylor and Taylor, 1997). This may result in nephritis, kidney damage, and an increased cancer risk. To ensure that there is insignificant risk to human health over a lifetime, the Environmental Protection Agency (EPA) regulation for public water systems sets the maximum contaminant level (MCL) for uranium at 30 μg/L, effective Dec. 8, 2003.
Methods for determining uranium levels in various types of samples have been developed. For instance, U.S. Pat. No. 4,198,568 to Robbins, et al. teaches uranium determination in aqueous samples through ultraviolet light-induced phosphorescence of the uranium. U.S. Pat. No. 5,190,881 to McKibbin teaches alpha-spectrometry methods to measure uranium content in biological fluids. U.S. Pat. No. 4,349,350 to Fitoussi, et al. teaches that uranium in an organic solvent can be determined by the addition of excess dialkyl dithiophosphoric acid to the organic solvent/uranium solution following which the uranium is converted to a mixed compound complex. The optical density of the solvent containing the complex is then measured to determine the concentration of the organic solvent and the uranium content in the complex. U.S. Pat. No. 6,107,098 to Kalinich discloses a method including mixing a uranium-containing biological sample with a buffer, one or more masking agents, and a solubilizing compound to form a uranium-containing metal binding complex composition. This composition is then combined with a pyridylazo indicator dye. The increase in absorbance due to the complexation of uranium with the dye is then determined with a spectrophotometer or a colorimeter.
Despite such advances in the art, groundwater analysis for uranium content can still be conducted only by a certified radiochemistry laboratory, of which there are very few (believed to be about five) in the United States. Problems with attempts to develop more cost effective, simple uranium detection methods have generally centered around two difficulties; the first being that methods require extensive sample preparation before analysis, and the second being that uranium determination procedures currently require elaborate instrumentation.
While the above-described methods and materials provide certain advances in the art, room for improvement and further advances exist. For instance, a consumer-based test to detect uranium in drinking/groundwater that is rapid, accurate, does not require extensive sample preparation, and requires little or no technical training to carry out would be of great benefit.